1. Field of the Invention
The present invention relates to a time base corrector for correcting a time base error, or a time base instability, of a signal such as a video signal reproduced from a magnetic tape.
2. Description of the Prior Art
In video tape recorders used for broadcast, time base correctors have been widely used for correcting time base errors of reproduced video signals.
U.S. Pat. No. 3,931,638 discloses a time base corrector employing analog shift registers in which one register has samples of one video line signal clocked into at a rate corresponding to its time base instability, and the other register clocks out, at a uniform rate, analog signals of the previous line, and vice versa. Another type time base corrector employing variable delay lines is disclosed in an article "Overview of Time-Base Correction Techniques and Their Applications" by K. Sadashige, SMPTE Journal Vol. 85 October 1976, pp. 787-791. These time base correctors have generally known disadvantages due to the use of analog devices such as analog shift registers and variable delay lines.
The above Sadashige's article disclose also, in pages 789-790, a time base corrector employing a digital memory. A similar digital time base corrector is disclosed in U.S. Pat. No. 4,120,000. These conventional digital time base correctors are basically configured as follows.
A reproduced analog video signal with a time base error is sampled and converted into a digital signal by an analog-to-digital (AD) converter responsive to a clock signal which is locked to the incomming reproduced analog video signal. The digital signal is stored into a memory. The temporarily stored digital signal is read out in response to another clock signal that has a fixed time base. The read-out digital signal is converted to an analog signal by a digital-to-analog (DA) converter. The analog video signal outputted from the DA converter has no time base errors.
Such conventional digital time base corrector, however, have some problems. One problem is that it must be provided with analog components in the clock generator for generating the clock signal which is locked to the incomming reproduced analog video signal. Performance variations and temperature characteristics of the analog components will cause undesired variations of the generated clock signal, which badly affect the performances of the time base corrector. In addition, since the analog components including capacitors and inductors cannot be mounted in an integrated circuit, it is difficult to reduce the cost of the time base corrector.
Another problem is that, since the memory writing and reading operations are controlled by the respective two different clock signals, constructions of the memories and memory controller are complicated.
Furthermore, since the AD converter is driven by the time base fluctuated clock signal, the output of the AD converter cannot be used commonly in other digital processing systems.